The present invention relates to a superimposing apparatus for displaying an original image signal overlapped with another image signal, and more particularly, to a superimposing apparatus for performing an image overlap or a letter mixture while maintaining the shape of the original image.
FIG. 1A is a block diagram showing a conventional superimposing apparatus for generating a color difference signal. In the drawing, a control signal SICSEL is for selecting a color signal to be used in an overlapping image or mixing title and a control signal SICBLK is for designating a position of the overlapping image or the mixing title within the image displayed on a screen.
The superimposing apparatus of the color difference signal includes an address decoder 1 for generating an address according to the control signal SICSEL, a first Read-Only-Memory (ROM) 2 for storing a plurality of Red-Yellow (R-Y) color difference signals and for outputting the R-Y color difference signal SIRY which corresponds to an input address, a second ROM 3 for storing a plurality of Blue-Yellow (B-Y) color difference signals and outputting the B-Y color difference signal SIBY which corresponds to the input address. The conventional superimposing apparatus of FIG. 1A also includes a first multiplexor (MUX) 4 for receiving a red-yellow chrominance signal CRY and the SIRY signal and for outputting one of the signals according to a position control signal SICBLK, a D flip-flop 5 for latching the output of the first MUX 4 according to a subclock SCK, a second MUX 6 for receiving a blue-yellow chrominance signal CBY and the SIBY signal and outputting one of the signals according to the position control signal SICBLK and a D flip-flop 7 for latching the output of the second MUX 6 according to the subclock SCK.
FIG. 1B is a block diagram showing a luminance signal superimposing apparatus according to a conventional technology. In the drawing, a control signal SIYSEL is for selecting the luminance signal to be used in the overlapping image or the mixing title, and a control signal SIYBLK is for designating the position of the overlapping image or the mixing title.
The luminance signal superimposing apparatus includes an address decoder 8 for generating an address according to the control signal SIYSEL, a third ROM 9 for storing a plurality of luminance signal levels and for outputting the luminance signal which corresponds to the input address, a delay unit 10 for delaying the luminance signal for a predetermined amount of time in order to synchronize the luminance signal and the color signal, a third MUX 11 for receiving the output of the delay unit 10 and the third ROM 9 and outputting one of them according to the position control signal SIYBLK, and a D flip-flop 12 for latching the output of the third MUX 11 according to a system clock PCK. Here, the frequency of the subclock SCK above is 1/2 that of a system clock PCK.
Generally, the conventional superimposing apparatus for overlapping a predetermined image or mixing the title with the original image in a recording device, such as a camcorder and a VCR, is constructed as shown in FIGS. 1A and 1B. In the conventional superimposing apparatus, the original image which exists in the overlapping position is lost by deleting the original image that coincides with the overlapping position on the screen when displaying the luminance signal and the color signal of the overlapping image which is read from the ROMs 2, 3 and 9 in the position from which the original image was deleted. This is also the case when performing a title mix. That is to say that, when performing either an image overlap or a title mix with an original image signal, when a predetermined overlapping image signal is displayed in an overlapping position on the screen then the original image in the overlapping position is lost.